Toy Story (the original) remains a wonderful film in its own right, irrespective of the cultural and technological watershed it was - but I had to give Toy Story 2 the nod, since it's themes, story and script are nigh perfect (an astounding feat once you learn about the pressures that production was up against). Toy Story 4 has a special place in my heart - I think it's the most "grown-up" movie of the franchise, and I'm partial to the love story between Bo and Woody. Toy Story 3 is a gem of a film as well, but even though it came out around the same time I was going to college myself, and it's certainly a very strong film, it just didn't touch me the way the others have. Of the four Toy Story films, there's not a bad one in the bunch.
I haven't seen Lightyear yet.